Connecting the Dots: Building Spectroscopic Blocks

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The arrival of LASER has ushered a new revolution in the field of spectroscopy. Several nonlinear spectroscopic experiments based on multiple pulse irradiation have been designed and successfully implemented. Nevertheless, the interest of chemists lies predominantly in understanding chemical reactions in bulk and at interfaces. At Paderborn university, we had been working on to provide a thorough understanding of the ‘On-water catalysis’. As theoretical chemists, we were posed with the question if we could provide a simple computational spectroscopic design to study chemical reactions on the interfaces.

This task involved merging two independent models or in the layman’s terminology ‘connecting the dots’. We had recently established a simple linear relationship between the hydrogen bond strength and the stretching mode of water molecules (Ojha et al. 2018). These linear relations let us selectively pick water molecules with a specific stretching frequency or in other words, "pump" water molecules to a given vibrational level. They also let us design the pulse with the width of our choice corresponding to both the broadband and narrow pulse excitation. We used this model in combination with the surface-specific velocity based sum-frequency generation spectroscopy (SFG) method (Ohto et al. 2015) to develop an elegant pulse sequence. The pulse sequence can be summarized as IR-pump SFG-probe spectroscopy. This combination lets us take a leap from conventional time-averaged SFG to a paradigm of time-dependent SFG at no extra computational cost. Moreover, it also lets you circumvent the tedious response functions calculation which is needed to obtain time-resolved two-dimensional SFG. In a nutshell, our model of time-dependent SFG could be used to study the dynamics of generic surface-specific chemical processes.

On the one hand,  the water molecules orientated towards the surface which are not hydrogen bonded have a greater propensity to form a hydrogen bond. On the other hand, a hydrogen-bonded water molecule pointing towards bulk takes more time to break a hydrogen bond. We used our computational time-dependent SFG to quantitatively establish this conventional wisdom.  The computational implementation of time-dependent SFG opens the avenues for the development of new and more complex pulse sequences. As future work, we are looking forward to studying prototype reactions at the aqueous-organic interface using this method. While we have only connected two dots, a lot remains to be explored! Read the complete story at Communication Chemistry.


1. Ojha et al.  Sci. Rep. 8, 16888 (2018)

 2. Ohto et al.  J. Chem. Phys. 143, 124702 (2015)


Deepak Ojha

Postdoctoral Associate, Paderborn Universität

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